The Body’s Defense Against Pathogens and Bacteria

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How does the human body defend against pathogens and bacteria? What strategies and counters do the body use?

Some bodily defense mechanisms have dual purposes that benefit both the host and the pathogen. Similarly, some infectious diseases cause damage to the host which is costly to the pathogen as well. This is because both the host and the pathogen need to make compromises.

Keep reading to learn about the body’s defense against pathogens and bacteria.

Nuances of the Body’s Defense System Against Pathogens

Less accessible body areas have less regenerative capability. Infections of the brain or heart are usually fatal, so maintaining regenerative capabilities in these areas would have little benefit. As a result, natural selection seems to have shown that defense against pathogens and bacteria to protect the brain during the rare infection don’t outweigh the costs of maintaining this system.

Some defensive behaviors have dual purposes, benefiting both host and pathogen.

  • Sneezing and diarrhea may be a defensive mechanism, but it also helps spread the pathogen.
  • Ideally, in medicine, we fine-tune defense to net benefit the human host.

Sometimes, infectious disease really does cause damage to the host. Often this is done to procure more resources for the pathogen and support its spreading or reproduction. Beyond these purposes, damage to the host is often incidental and costly to both host and pathogen. It does no good for the tapeworm to cause its host to be malnourished; it does no good for hepatitis to destroy the liver completely and quickly kill the host. Any especially virulent strain that rapidly killed its host would have little chance to spread itself, and the mutations that caused its virulence would be selected against.

Pathogens Adapt to Hosts

Just like animals, bacteria and viruses undergo natural selection to propagate their genes. The pathogens that can best overcome host defenses succeed in reproducing and spread their beneficial genes to the population.

Pathogens have evolved a variety of responses to host defense, including:

  • Imitating the host immune system. 
    • Streptococcus bacterium presents a surface so similar to human cells that the immune system has difficulty recognizing it. In fact, antibodies produced in response to strep can damage the host itself, causing rheumatic fever and even mental disease like Sydenham’s chorea and possibly obsessive-compulsive disorder.
  • Evading the immune system.
    • The parasite that causes African sleeping sickness changes its surface proteins constantly to evade detection by antibodies. This is like a spy constantly changing its disguise.
  • Manipulating the host.
    • The rabies virus moves to the brain and adjusts the host behavior to better increase its own propagation. It paralyzes swallowing muscles, causing saliva to build up in the mouth and thus increasing the concentration of virus in the mouth. Then it increases the host’s aggression to better spread by bite.

Strategies and Counters

We’ve seen a variety of strategies for hosts and counterstrategies by pathogens. Some observations of infectious disease, like decreasing iron levels, benefit the host; others benefit the pathogen; some are merely incidental damage in the ongoing war.

Here is a classification of how infectious disease manifests, based on their function.

ObservationExamplesBeneficiary
Hygienic measure by hostKilling mosquitoes, avoiding sick neighbors, grooming, removal of parasitesHost
Host defensesFever, iron withholding, sneezing, vomitingHost
Repair of damage by hostRegeneration of tissuesHost
Compensation for damage by hostChewing on the other side of the mouth to avoid tooth pain from infected toothHost
Damage to host tissues by pathogenTooth decay, hepatitis liver damageNeither
Impairment of host by pathogenIneffective chewing, decreased detoxification of blood by infected hepatitis liverNeither
Evasion of host defenses by pathogenMolecular mimicry (MHC complex), change in antigens (trypanosome changes surface proteins)Pathogen
Attack on host defenses by pathogenDestruction of white blood cells; secretion of factors that inhibit inflammationPathogen
Uptake and use of nutrients by pathogenGrowth and proliferation of trypanosomes, viruses bind to cellular receptors and enter cells to hijack resources and replicatePathogen
Dispersal of pathogenTransfer of malaria parasite to a new host by mosquito, sneezingPathogen
Manipulation of host by pathogenExaggerated sneezing, diarrhea, behavioral changes (rabies increases chance of bites)Pathogen
The Body’s Defense Against Pathogens and Bacteria

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Hannah Aster

Hannah graduated summa cum laude with a degree in English and double minors in Professional Writing and Creative Writing. She grew up reading books like Harry Potter and His Dark Materials and has always carried a passion for fiction. However, Hannah transitioned to non-fiction writing when she started her travel website in 2018 and now enjoys sharing travel guides and trying to inspire others to see the world.

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